News Release Number: STScI-2006-06

Astronomers Find Smallest Extrasolar Planet Yet Around Normal Star

The full news release story:

Using an armada of telescopes, an international team of astronomers has
found the smallest planet ever detected around a normal star outside
our solar system.

The extrasolar planet is five times as massive as Earth and orbits a
red dwarf, a relatively cool star, every 10 years. The distance between
the planet, designated OGLE-2005-BLG-390Lb, and its host is about three
times greater than that between the Earth and the Sun. The planet's
large orbit and its dim parent star make its likely surface temperature
a frigid minus 364 degrees Fahrenheit (minus 220 degrees Celsius). This
temperature is similar to that of Pluto, but the newly found planet is
about one-tenth closer to its star than Pluto is to the Sun.

Its detection, however, opens a new window in the search for Earth-like
worlds.

"This finding means that Earth-mass planets are not that uncommon," said
Kailash Sahu of the Space Telescope Science Institute in Baltimore, Md.,
and a founding member of the Probing Lensing Anomalies Network team
(PLANET) that helped detect the new planet. "If we found one, there must
be more."

The result will be reported tomorrow in a Letter to the journal Nature.

The finding also supports theories for how our solar system formed.
"The favored theory proposes that planets were created from material
accreting around a star," explained Bohdan Paczynski of Princeton
University and a member of the Optical Gravitational Lensing Experiment
(OGLE), a group that also helped to discover the planet. Paczynski and
Shude Mao proposed the idea of using gravitational microlensing to
discover planets in 1991. "Around red dwarfs, the theory predicts Earth-
and Neptune-sized planets to be more common than Jupiter-sized planets.
The planets would be located between 0.1 and 10 times the Earth-Sun
distance from their stars."

Astronomers discovered the planet indirectly with a technique called
gravitational microlensing. The technique takes advantage of the random
motions of stars, which are generally too small to be noticed. If one
star, however, passes precisely in front of another star, the gravity
of the foreground ("lens") star bends the light from the background
("source") star. The foreground star, therefore, acts like a giant lens,
amplifying the light from the background star, a phenomenon called
gravitational microlensing. A planetary companion around the foreground
star can produce additional brightening of the background star. This
additional brightening can thus reveal the planet, which is otherwise too
faint to be seen by telescopes.

The higher the mass of the "lensing" star, the longer is the duration
of the microlensing event. So, while a microlensing event due to a star
lasts many days, the extra brightening due to a planet lasts a few
hours to a couple of days. In the case of the newly found planet, the
extra brightening lasted only about 12 hours.

Using the microlensing technique, astronomers determined the planet's
mass. This method, however, does not reveal any clues about an object's
composition. Astronomers think the planet is composed of ice and rock.
Its estimated mass suggests that it is a giant version of terrestrial
planets like Earth and Mars. The planet orbits the most common star in
our Milky Way Galaxy, a red dwarf five times less massive than the Sun.
The pair is located about 20,000 light-years away in the constellation
Scorpius, not far from the central bulge of our galaxy.

None of the roughly 160 planets found outside our solar system has
been imaged directly. The planets are too dim and too close to their stars to
be seen. Astronomers have discovered most of them by detecting the
gravitational tugs the unseen planets exert on their parent stars. This
popular technique, however, favors finding large planets orbiting very
close to their hosts.

"Microlensing is a promising technique to find Earth-mass planets
because other current planet-hunting techniques are not sensitive to
discovering low-mass planets like Earth," Sahu explained.

Because microlensing events are unpredictable and rare, astronomers
improve their chances of observing one by looking at many stars at
once. To catch a microlensing event, monitoring teams such as OGLE
watch 100 million stars every night in the crowded bulge of our galaxy.

In the past decade, the OGLE team has found more than 1,000 microlensing
events. OGLE's monitoring team, however, may not notice the
short-duration brightening caused by a planet.

PLANET has a dedicated army of 1-meter class telescopes, including the
European Space Observatory's Danish 1.54-meter telescope at La Silla in
Chile, to specifically look for planetary signatures through continuous
monitoring of ongoing microlensing events. The group has monitored about
200 microlensing events during its 10-year campaign. So far, three
planets have been discovered with gravitational microlensing, including
OGLE-2005-BLG-390Lb. The newly found planet is the smallest of the three
and marks the first planet found by the PLANET team. To increase its
chances of finding planets, the PLANET team joined forces in 2005 with
RoboNet, a network of 2-meter robotic telescopes operated by the United
Kingdom.

PLANET, RoboNet, OGLE, and other monitoring teams discovered the planet
in 2005. The microlensing event was first spotted on June 11, 2005, by
the OGLE search team. The OGLE-2005-BLG-390 event triggered the PLANET
telescopes to start collecting data. On Aug. 10, round-the-clock
monitoring by the PLANET team revealed an additional brightening that
led to the discovery of the planet. An OGLE telescope from the same
night also detected the planetary signature. The Microlensing
Observations in Astrophysics (MOA) team later identified the background source star on its images and also confirmed the planetary signature.